A waveguide may serve as an optical amplifier by doping it with ions of a rare earth element such as Erbium. An optical signal propagating in the waveguide is amplified when a pumping light beam is introduced. For example, Erbium ions, excited to a higher energy state with a pumping light beam having a wavelength of approximately 980 nm or 1480 nm, will amplify an optical signal in a wide wavelength band around 1530-1600 nm as the Erbium ions fall down to a lower energy state. This technique is well-known in optical fiber amplification.
FIG. 1 is a schematic diagram showing one prior art method of amplifying an optical signal 10 in a planar waveguide 20. The waveguide 20 is embedded in a substrate 30 and doped with Erbium ions. An optical signal 10 is directed into the waveguide 20 and propagates through the waveguide 20. A laser 50 supplies pumping light beams into the waveguide 20 in a co-propagating direction, i.e., in substantially the same direction as the optical signal propagates. The signal 10 and the pump 50 are combined to the same waveguide 20, for example, in an evanescent directional coupler. In one example, an optical signal 10 having wavelength of approximately 1550 nm is amplified as laser 50 supplies pumping light beams of approximately 980 nm or 1480 nm wavelength.
FIG. 2 is a schematic diagram showing another prior art method of amplifying an optical signal. In FIG. 2, a pump laser 50 is directed from the opposite end of the waveguide 20 to pump light in a counter-propagating direction, i.e., in a direction opposite to that of the optical signal. Similar to FIG. 1, the optical signal is amplified within the waveguide 20 and then exits the substrate 30.
Modern optical networks use single-mode optical fibers for transmission over long distances. This avoids signal degradation coming from chromatic dispersion, i.e. dependence of the speed of the light on its wavelength. For efficient interfacing with single mode fibers, all optical components, including fiber or waveguide amplifiers, are effectively single-mode. Due to a general principle of optics, “brightness conservation theorem”, power of light in a single mode cannot be increased using just linear passive (not adding energy) optical elements. This results in a fact that the power of light with a certain wavelength from only one mode can be coupled to a single mode waveguide. For amplifiers, it translates that only one pump laser with a certain wavelength can supply pump light in each direction of propagation and each polarization.
The optical signal experiences gain in an optical amplifier provided that the intensity of the pump is higher than a certain threshold value dependent on the intensity of the optical signal and material properties of the optical amplifier. In order to achieve high enough gain, the intensity of the pump must be much higher than the threshold value. Consequently, a high power of a pump laser is typically required.
There are several disadvantages of the above methods compared to the invention described below. First, the relatively high power laser used in the described co-propagating and counter-propagating amplification is expensive. Second, high power lasers have a high power dissipation, which may cause thermal issues in their packaging. Third, the reliability of high power lasers is generally not as good as that of lower power lasers.